diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 07eaabc9..edbfb51e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -27,11 +27,32 @@ jobs:
       - run: nix build -L .#aeneas
       - run: nix build -L .#checks.x86_64-linux.default
 
+  diff_lean_files:
+    runs-on: [self-hosted, linux, nix]
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0 # deep clone in order to get access to other commits
+      - id: skip_check
+        run: |
+          # Compares this version with `main`, and checks whether the lean files changed.
+          if git diff origin/main --quiet -- tests/lean && git diff origin/main --quiet -- backends/lean; then
+              echo 'lean files were not changed in this PR.'
+              echo "CHECK_LEAN=false" >> "$GITHUB_ENV"
+          else
+              echo 'lean files were changed in this PR.'
+              echo "CHECK_LEAN=true" >> "$GITHUB_ENV"
+          fi
+    outputs:
+      check_lean: ${{ env.CHECK_LEAN }}
+
   lean:
+    needs: [diff_lean_files]
     runs-on: [self-hosted, linux, nix]
+    if: needs.diff_lean_files.outputs.check_lean == 'true'
     steps:
-      # Lean cannot run its tests in the sandbox because `elan` will download things
       - uses: actions/checkout@v4
+      # Lean cannot run its tests in the nix sandbox because `elan` will download things
       - run: nix develop --command bash -c "cd tests/lean && make"
 
   charon-pin-is-forward:
diff --git a/flake.lock b/flake.lock
index 8f4bd0ae..19869f91 100644
--- a/flake.lock
+++ b/flake.lock
@@ -55,21 +55,6 @@
         "type": "github"
       }
     },
-    "flake-compat_2": {
-      "locked": {
-        "lastModified": 1688025799,
-        "narHash": "sha256-ktpB4dRtnksm9F5WawoIkEneh1nrEvuxb5lJFt1iOyw=",
-        "owner": "nix-community",
-        "repo": "flake-compat",
-        "rev": "8bf105319d44f6b9f0d764efa4fdef9f1cc9ba1c",
-        "type": "github"
-      },
-      "original": {
-        "owner": "nix-community",
-        "repo": "flake-compat",
-        "type": "github"
-      }
-    },
     "flake-utils": {
       "inputs": {
         "systems": "systems"
@@ -173,7 +158,6 @@
     "root": {
       "inputs": {
         "charon": "charon",
-        "flake-compat": "flake-compat_2",
         "flake-utils": [
           "charon",
           "flake-utils"